The American Heart Association indicates that 30,000 cardiopulmonary bypass (CPB) surgeries were done on patients of ages less than 15 years in the USA in 2002. Of these cases, 18,000 were specifically for the repair of congenital defects of the heart. Over the past decade, mortality rates associated with pediatric cardiopulmonary bypass procedures have been significantly reduced, yet morbidity remains a major clinical problem with patients suffering cerebral, myocardial, or renal dysfunction following CPB. Factors associated with extracorporeal circuit (ECC) technology, such as non-pulsatile perfusion, hemodilution, and acute injury due to mishap have been implicated in patient morbidity. Despite this evidence, innovation has been slow in coming. Roller pumps that were first used in perfusion studies in 1935 are still relied on in 98% of centers performing pediatric CPB today.
One visible side-effect of CPB in infants and children is systemic accumulation of edema fluid. In a prospective study of 100 neonates undergoing corrective cardiac surgery employing approximately 2 hours of CPB, the average fluid accumulation was greater than 600 ml. Much of this phenomenon is related to the hemodilution and foreign surface exposure of the blood loop. A typical infant of 3.5 kg weight has an estimated blood volume of 280 ml, and the extracorporeal loop with the venous reservoir, oxygenator, blood filter, and tubing can easily reach 700-800 ml of prime, resulting in a dilution factor of 2.5:1 to 3:1. Hemodilution in infants can far exceed that seen in adult patients, where 25% to 33% dilution rates are typical. Hemodilution results in lower hematocrit with associated reduction of oxygen delivery capacity, and is associated with a higher transfusion rate and increased use of all blood products with concomitant infection risk.
Deep hypothermic circulatory arrest is commonly used in the repair of congenital defects of the heart. Cessation of blood flow to the collateral circulation allows the surgeon to properly visualize the surgical field, while hypothermia reduces metabolism providing cellular protection despite lack of oxygen delivery. In recent practice, deep hypothermic circulatory arrest is conducted with intermittent periods of very low blood flow in the range of 10 to 20 cc/kg/min or “trickle flow”. It is commonly felt that this amount of flow can be provided without compromising the conduct of the surgical repairs, and will serve to preserve brain high energy phosphate concentrations and intracellular pH (20). In order to meet these requirements the arterial pump must be capable of maintaining flow accuracy over a broad range of flow rates and temperature from 10 cc/kg at 15° C., to 150 cc/kg at 37° C.
Centrifugal pumps are simply not practical in providing for extreme low flow rates due to excessive impeller speeds and resulting blood damage and in fact are relied on only in 2% of centers conducting pediatric heart surgery. Occlusive roller pumps are currently used; however, they are far from optimal in their use at low flow rates.
Generally, roller pumps rely on a roller pressing against a piece of tubing backed by a rigid raceway. In order to fully occlude the circular tubing for use in very low flow conditions, excessive roller forces are needed to squeeze the tubing between the roller and raceway. This significantly increases stress and wear on the tubing, potentially causing leaks or ruptures. A review of the Manufacturer and User Facility Device Experience Database (MAUDE) reports supports the conclusion that tubing leaks and rupture are common events with potentially injurious results.
Traditionally, roller pumps provided no inherent means of preventing draining of the venous reservoir, and if left unattended, would drain the reservoir and continue to pump air to the patient until rotation was halted. A minimum “safety volume” of blood had to be maintained in the reservoir when using a roller pump so as to provide sufficient time for the perfusionist to react to sudden interruptions of venous return flow before the reservoir was drained. For example, at a flow rate of 1.5 l/min, using the known state-of-the-art Terumo Capiox reservoir, 300 ml of reservoir volume would provide less than 12 seconds of response time.
This has prompted the use of reservoir level detectors and air detectors with pump shut off interconnections. 79.2% of centers conducting pediatric extracorporeal circulation (ECC) utilize reservoir level detectors, and 87.5% of these centers utilize air bubble detectors. However, despite their presence, these safety devices may fail to protect due to device failures and human errors. In practice, a typical circuit volume for a small infant could range from 600-800 ml.
In order to provide a safe operational venous reservoir level for use with roller pumps, 200 additional ml are typically added to the circuit, which is usually whole blood or packed red blood cells. This safety volume is highly variable amongst practitioners and could be minimized if a self-limiting safety system was designed into the pump. If this 200 ml volume could be eliminated, the savings in both hemodilution side-effects and risks to additional blood product transfusion would be of significant benefit.
Proper setting of the degree to which a roller pump occludes the tubing is also critical. If there is too little occlusion, the pump fails to create sufficient flow. Over occlusion creates excessive stress in the tubing which can lead to splitting with subsequent blood loss and air introduction to the arterial circulation. Split tubing continues to be a common problem with traditional roller pumps.
Current peristaltic pump technology typically operates with two pump rollers and a 180 degree arc over which the pump tubing is occluded by the rollers and a stator. In this design, fluid enters the pump tubing from the venous reservoir under low pressure head conditions, typically 50 mmHg or less. The purpose of the roller pump is to shuttle this fluid from the inlet to the outlet and force it to flow through the tubing circuit. Typically in heart surgery this involves moving blood from a low pressure inlet to a high pressure outlet. As the roller head (rotor) turns, a roller contacts and advances along the tubing filling it with low pressure blood. At approximately the 180 degree point of the stator arc, a second roller contacts the tubing and isolates the fluid between the rollers still at the low inlet pressure. This situation lasts only briefly as the first roller departs from the tubing exposing the low pressure isolated fluid to the high pressure outlet fluid. This causes an equilibration of pressure between the fluid volumes and is associated with a momentary drop in pressure in the outlet. As the second roller continues to advance it drives the fluid forward reestablishing pressure within the outlet tubing.
Another style of roller pump, without a stator, utilizes a roller head (rotor) with three rollers and a conduit having an occlusive portion. The conduit extends around the rollers. The occlusive portion remains occluded as long as the pressure on the outside of the conduit is equal to or greater than the pressure on the inside of the conduit. When the fluid inlet supply pressure exceeds the pressure acting on the exterior of the conduit, the occlusive segment will inflate and fill with fluid and the pump will force the fluid through the outlet of the conduit. Such a pump is described in more detail in U.S. Pat. No. 5,486,099, which is herein incorporated by reference.
There is an ongoing debate over pulsatile versus non-pulsatile circulatory support. Various published studies, however, have substantiated some advantages with pulsatile support, especially as it relates to cardiopulmonary support. These studies indicate that pulsatile support reduces systemic vascular resistance and attenuates the catecholamine response, improves myocardial blood flow, and improves overall clinical outcomes. Cerebral pressure-flow auto-regulation has been proven to be intact in adult patients when the mean arterial pressures (MAPS) were greater than 50 mmHg. However, for pediatric patients, where MAP often ranges between 20 to 40 mmHg before and after deep hypothermic cardiac arrest, pulsatile perfusion becomes important for maintaining cerebral blood flow.
Conventional roller pumps can be used to create pulsatile flow and pressure by rapidly accelerating the speed, revolutions per minute (RPM), of the rotor for a “systolic” period and reducing the speed (RPM) to create a “diastolic” period. This has significant disadvantages as it involves use of much greater power to accelerate the rotating mass, increases tubing wear, and increases blood exposure to damaging negative pressures. With this technique it is not possible to isolate the inlet conditions from the outlet conditions. Additionally, the inlet conditions vary as the speed (RPM) is modulated.
In view of the above limitations and drawbacks of the known technology, it is seen that there is a need for a ventricular roller pump that provides pulsatile pressure and flow profiles having amplitudes and rise times that approximate those of a human heart, while maintaining a constant speed (RPM).